The present invention relates to a fuel cell system with a fuel cell unit that includes a metering unit with at least one metering valve for metering the fuel, hydrogen in particular.
Modern fuel cell systems for stationary and mobile applications are generally operated such that the anode side is supplied with fuel, hydrogen in particular, and the cathode side is supplied with oxygen, air in particular. The incoming and outgoing gas flows are pumped or blown and released through the fuel cell stack.
At times, a relatively high portion of unconverted hydrogen is blown off of the anode side of the stack in particular. At times, the hydrogen-rich anode exhaust gas is pumped or recirculated in the circuit, to improve the overall efficiency of the system.
In addition, the cathode gases, at the least, are typically humidified before they enter the fuel cell stack, to prevent the proton-conducting membrane of the stack from drying out. Membranes or the MEA (membrane electrode assembly) of this type must have a certain minimum moisture level in order to conduct the protons.
To supply fuel to the fuel cell system, the flow of fuel or hydrogen and atmospheric oxygen must be supplied to the fuel cell stacks on an as-needed basis. Systems of this type may have various designs. For example, metering systems with one or more metering valves are provided, to ensure the largest metering range possible combined with a relatively low tolerance in terms of the quantity to be metered.
In addition, gas-supply systems are already known, in the case of which the fuel cell stack is supplied with the required quantity of fuel or hydrogen, and, in a start phase, both the anode and the cathode of the stack are supplied with hydrogen or fuel. The latter is realized in order to warm the cold fuel cell stack to the necessary and/or optimal operating temperature as quickly as possible. When heat is added, the atmospheric oxygen present in the fuel cell stack reacts with the fuel or hydrogen, thereby releasing heat energy.
The cathode of the fuel cell stack requires extremely small quantities of hydrogen in the start phase. The start valve must therefore be designed small in size, which means that specially developed start valves must be used. This results in relatively high costs, however.